Projection resistance brazing of superalloys

ABSTRACT

Superalloy components are joined by mating a recess formed in one component with a corresponding projection formed in another component along a contact surface. The components are compressed along the contact surface and resistance heat brazed to each other. Current is passed between the components at a selected flow rate and application time until brazing alloy melting occurs along the contact surface, and they are mutually affixed to each other. When repairing a damaged surface portion of a superalloy material component, the damaged portion is removed to form an excavated recess. A repair splice is formed, preferably of a same material with similar mechanical structural properties, having a mating projection with profile conforming to the corresponding recess profile. The splice and substrate are resistance heat brazed under compression pressure until brazing alloy melting occurs along the contact surface, so that they are mutually affixed.

CLAIM TO PRIORITY

This application claims the benefit of U.S. provisional patentapplication entitled “Projection Resistance Brazing of Superalloys forRepair of Service-Degraded Components”, filed Nov. 7, 2011, and assignedSer. No. 61/556391, which is incorporated by reference herein.

BACKGROUND OF THE DISCLOSURE

1. Field of the Invention

The invention relates to structural joining of advanced superalloycomponents during fabrication and/or repair. In some embodiments, theinvention relates to surface repair of superalloy turbine blades andvanes in steam or gas turbines, by use of splice inserts that areaffixed to a new or repaired substrate by projection resistance heatbrazing under contact pressure, in a manner that does not significantlyreduce mechanical structural or material properties of the joinedcomponents.

2. Description of the Prior Art

“Structural” repair of gas turbine or other superalloy components iscommonly recognized as replacing damaged material with matching alloymaterial and achieving properties, such as strength, that are close tothe original manufacture component specifications (e.g., seventy percentultimate tensile strength of the original specification). For example,it is preferable to perform structural repairs on turbine blades thathave experienced surface cracks, so that risk of further cracking isreduced, and the blades are restored to original material structural anddimensional specifications.

Structural repair or new fabrication of nickel and cobalt basedsuperalloy material that is used to manufacture turbine components, suchas cast turbine blades, is challenging, due to the metallurgicproperties of the finished blade material. For example, as shown in FIG.1, a superalloy having more than 6% aggregate aluminum or titaniumcontent, such as CM247 alloy, is more susceptible to strain age crackingwhen subjected to high-temperature welding than a loweraluminum-titanium content X-750 superalloy. The finished turbine bladealloys are typically strengthened during post casting heat treatmentswhich render them difficult to perform subsequent structural welding.Currently used welding processes for superalloy structural fabricationor repair generally involve substantial melting of the substrateadjoining the weld preparation, and complete melting of the welding rodor other filler material added. When a blade constructed of such amaterial is welded with rods of the same or similar alloy, the blade issusceptible to solidification (aka liquation) cracking within andproximate to the weld, and/or strain age (aka reheat) cracking duringsubsequent heat treatment processes intended to restore the superalloyoriginal strength and other material properties comparable to a newcomponent.

A past attempt to perform traditional “spot” electric resistance weldingof superalloys, in a heat resistance joining apparatus, by passingcurrent between compressed electrodes into a pair of abutting superalloycomponents also caused solidification cracking within the weld zone.Alternative superalloy welding processes, including laser microcladdingwith chill fixtures, welding in so-called “hot” boxes at elevatedtemperatures, and inertia friction welding may still lead to post weldheat treatment strain age cracking. Other friction welding processes,such as friction stir welding, can reduce superalloy crackingpropensity, but the employed welding apparatus has relatively limitedtool life. The same cracking concerns occur during superalloy componentfabrication, when separate components constructed of superalloy materialare joined by welding processes.

In comparison to structural repair or fabrication, “cosmetic” repair orfabrication of superalloys is recognized as replacing damaged material(or joining two components of newly fabricated material) with unmatchingalloy material of lesser structural property specifications, where thelocalized original structural performance is not needed. For example,cosmetic repair may be used in order to restore the repaired component'soriginal profile geometry. As noted above, it is desirable to performstructural repairs on surface cracks in order to reduce their likelihoodof subsequent spreading when the component is returned to service.Conversely, an example of cosmetic repair is for filling surface pits(as opposed to structural cracks) on a turbine blade airfoil in order torestore its original aerodynamic profile, where the blade's localizedexterior surface is not critical for structural integrity of the entireblade. Cosmetic repair or fabrication is often achieved by usingoxidation resistant weld or braze alloys of lower strength than theblade body superalloy substrate, but having higher ductility and lowerapplication temperature that does not negatively impact the superalloysubstrate's material properties.

In the past, electric resistance brazing has been commonly used forcosmetic metal joining of common ferrous and non-ferrous (e.g., copper)alloys. As shown in FIG. 2, brazing is performed by passing currentbetween compressed electrodes 22, 24 into a pair of abutting components30, 32. Solid sheet, powder or paste brazing alloy 34 is interposedbetween the components 30, 32. As electric current is passed between theelectrodes 22, 24 the brazing alloy 34 melts and affixes the commonferrous or non-ferrous alloy components 30, 32 to each other. However,the present inventor has no knowledge of past attempts to joinsuperalloy components by a resistance brazing process. Moreover, fillinga superalloy component surface crack with braze alloy may not beconsidered a sufficiently conservative repair approach to avoid futurecrack growth when the component is returned into service.

Given the shortcomings of superalloy structural repair welding, oftenthe only commercially acceptable solution is to scrap damaged turbineblades that require structural repair, because past experience has shownlimited success of such structural repairs. Thus repairs have beenlimited to those that have in the past been proven to be performedsuccessfully by alternative superalloy welding processes describedabove, or by cosmetic welding, employing more ductile welding rod fillermaterials with reduced structural strength.

Thus, a need exists in the art for a method for performing structuraljoining or repairs on surfaces of superalloy components, such as turbinevanes and blades, so that subcomponents can be joined; or thatstructural cracks and other surface defects can be repaired.

Another need exists in the art to increase successful rates ofstructural repairs on surfaces of superalloy components, such as turbinevanes and blades, so that damaged blade scrap rates can be reduced.

Yet another need exists in the art for a method for performingstructural joining or repairs on surfaces of superalloy components, suchas turbine vanes and blades, with proven, repeatable repair techniquesand machinery, that do not require complex welding or post-repair heattreatment procedures.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is to perform structural joiningor repairs on surfaces of superalloy components, such as turbine vanesand blades, so components can be joined; or that structural cracks andother surface defects can be repaired.

Another object of the present invention is to increase the likelihood ofperforming successful structural repair of superalloy components, suchas turbine vanes and blades, so that damaged component scrap rates canbe reduced.

Yet another object of the present invention is to perform structuralfabrication or repair of superalloy components, such as turbine bladesor vanes, with proven, repeatable repair techniques and machinery thatdo not require complex welding or post-repair heat treatment procedures.

These and other objects are achieved in accordance with the presentinvention embodiments by structurally joining superalloy components, orrepairing defects in superalloy material components, such as turbineblades or vanes. In repair embodiments, the surface defect is removedfrom the component substrate by electric discharge machining or otherknown metal working process, forming an excavated recess. A repairsplice is formed, preferably of a same material with similar mechanicalstructural properties, having a mating projection with profileconforming to the corresponding recess profile. The repair splice isinserted and captured within the recess so that they abut each otheralong a contact surface. The splice and substrate are resistance heatedunder compression pressure until melting of braze alloy occurs along thecontact surface, so that they are mutually affixed. A repair alloy brazemust be interposed between the splice and recess substrate prior toresistance heating.

The present invention features a joined superalloy component including asuperalloy substrate defining a recess having a recess profile; and amating superalloy splice having a splice projection captured within thesubstrate recess, with a projection profile conforming with thesubstrate profile along a contact surface within the recess. Thesubstrate and splice are affixed to each other along the contact surfaceby the process of electric resistance brazing by interposing brazingalloy between the recess and repair splice along the contact surface.Next the substrate and splice projection are compressed together alongthe contact surface at a selected pressure. The substrate and splice arecontacted with separate conductive electric resistance brazingelectrodes. Current is passed through the substrate and spliceprojection between the electrodes at a selected flow rate andapplication time period until localized melting of the brazing alloyoccurs along the contact surface. Current flow is ceased after thesubstrate and splice projection are mutually affixed to each other.

The present invention features a method for joining superalloystructures by forming a recess in a superalloy component substratehaving a recess profile defined by the remaining substrate; and forminga mating superalloy splice having a splice projection, with a projectionprofile conforming with the substrate recess profile along acorresponding contact surface. Brazing alloy is interposed between therecess and repair splice along the contact surface. The splice isinserted and captured within the recess, so that the projection andrecess are in abutting contact along the contact surface. The substrateand splice projection are compressed together along the contact surfaceat a selected pressure. The substrate and splice are in contact withseparate conductive electric resistance brazing electrodes. Current ispassed through the substrate and splice projection between theelectrodes at a selected flow rate and application time period untilbrazing alloy localized melting occurs along the contact surface.Current flow ceases after the substrate and splice projection aremutually affixed to each other.

The present invention also features a method for repairing a superalloycomponent by removing a damaged portion of superalloy componentsubstrate and forming an excavated recess therein having a recessprofile defined by the remaining substrate. A mating superalloy repairsplice is formed, having a splice projection, with a projection profileconforming with the substrate recess profile along a correspondingcontact surface. Brazing alloy is interposed between the recess andrepair splice along the contact surface. The repair splice is insertedand captured within the recess, so that the projection and recess are inabutting contact along the contact surface. The substrate and spliceprojection are compressed together along the contact surface at aselected pressure with a pair of opposed electric resistance brazingelectrodes. Current is passed through the substrate and spliceprojection between the electrodes at a selected flow rate andapplication time period until brazing alloy localized melting occursalong the contact surface and they are mutually affixed to each other.

The objects and features of the present invention may be applied jointlyor severally in any combination or sub-combination by those skilled inthe art.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a chart showing superalloy strain age cracking susceptibilityduring welding, as a function of titanium and aluminum content in thealloy;

FIG. 2 is a schematic elevational view of prior art resistance heatbrazing apparatus and methods;

FIG. 3 is a schematic elevational view of a superalloy component havinga surface defect in need of repair;

FIG. 4 is a schematic elevational view of a surface defect in asuperalloy component being repaired in accordance with an embodiment ofthe present invention by excavating a damaged portion thereof;

FIG. 5 is a schematic elevational view of a surface defect in asuperalloy component being repaired in accordance with an embodiment ofthe present invention by replacing the damaged portion and replacing itwith a repair splice;

FIG. 6 is a schematic elevational view of a surface defect in asuperalloy component being repaired in accordance with an embodiment ofthe present invention by affixing the repair splice to the componentsubstrate by resistance heating the repair splice and substrate;

FIG. 7 is a detailed schematic elevational view of the resistanceheating interface of FIGS. 6 and 7; and

FIG. 8 is a schematic elevational view of a superalloy componentrepaired in accordance with an embodiment of the present invention.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

After considering the following description, those skilled in the artwill clearly realize that the teachings of the present invention can bereadily utilized for mechanical structural joining/repair of superalloycomponents. In some embodiments, the teachings of the present inventioncan be readily utilized for structural repair of superalloy materialturbine blades and vanes of the type used in steam or gas turbines, byuse of splice inserts, in a manner that does not significantly reducemechanical structural or material properties of the joined or repairedblade. In the present repair method, the damage is removed, forming anexcavated recess. A repair splice is formed of a same material withsimilar mechanical structural properties, having a mating outer profileconforming to the corresponding recess profile. The repair splice isinserted and captured within the recess, so that the blade body andrepair splice are in abutting contact along a contact surface. Thesplice and abutting substrate are compressed under pressure and locallyheated with an electric resistance heating brazing apparatus, untilmelting of braze alloy occurs along the contact surface, so that theyare mutually affixed. The repaired component's overall mechanicalstructural properties are similar to those of an undamaged component.The repair methods of the present invention do not require complexbrazing, welding or heat treatment procedures, and use known electricresistance heating brazing equipment and affixation processes. Inaddition to performing repairs, the joining methods of the presentinvention may be used to fabricate superalloy structures by joiningsubcomponents.

Present Invention Mechanical Structural Joining/Repair

FIG. 3 shows an exemplary superalloy component 40 having surface stresscracks 42. If the stress cracks are not structurally repaired (i.e., bymere cosmetic repair with relatively softer, lower applicationtemperature welding or brazing alloys) there is a possibility that thecosmetically repaired cracks will re-crack and/or continue to spreadwithin the component substrate.

In the repair method of the present invention cracked regions 42 of thecomponent substrate 40 are excavated by electric discharge machining(EDM) or other known metal removal processes, leaving a recess 44 in theuncracked substrate, as shown in FIG. 4. The recess 44 profile isdefined by the remaining margins of the uncracked substrate 40. While atrapezoidal cross-sectional profile recess 44 is shown in FIG. 4, othercross-sectional profile configurations may be utilized, such asvee-shaped or arcuate-shaped profiles formed by cutting tool heads orEDM. The recess length (normal dimension in and out of the figure) maybe varied. Suitable plan view profiles for recess 44 include circularsymmetrical, square and rectangular profiles. When fabricating a newcomposite superalloy structure, two superalloy subcomponents arefabricated with mating recess and projection profiles and structurallyjoined under pressure, with heat being applied to the contact surfacesof both subcomponents with an electric resistance brazing apparatus. Itis also possible to fabricate a composite structure that does not havemating recess and projection portions by abutting two componentsrelative to each other without any mating projection and recess.

In known repair or fabrication methods, recess 44 would have been filledby heat application of softer filler material (cosmetic repair) or byfiller material of similar hardness. As previously discussed, otherwelding techniques applying superalloy filler generally induceundesirable cracks in the superalloy component during welding or duringsubsequent post-welding heat treatment.

The present invention differs from prior known replacement of superalloymaterial in cracks or formed recesses by inserting a repair splice 50,preferably constructed of the same superalloy material with similarmaterial properties as the repaired component, as shown in FIG. 5. Therepair splice 50 has a projecting portion 52 with a profile thatconforms and mates with the recess profile 44. As discussed above, newsubcomponents having mating projection and recess portions may be joinedto fabricate a new component by the same method.

A bonding filler braze alloy 60 must be interposed between the matingrecess 44 and projection 52 to aid their respective joining duringsubsequent resistance heating processes. Filler alloy is chosen forcompatibility with the chosen resistance heating bonding/joiningprocess. For example, brazing filler alloy, having a lower meltingtemperature than the superalloy substrate 40 and repair splice 50, ischosen for brazing processes. Welding filler alloys likely will havehigher melting temperatures. While powdered bonding filler 60 is shownschematically in FIG. 5, other forms of known filler may be applied,including by way of non-limiting example solid preshaped/preformed ring,foil or ribbon filler, granular filler, filler presintered on thesurface, or filler paste. Filler alloy dimensions (e.g., powder meshsize) may be varied to optimize resistance heating heat transfer fluxconcentration, in conjunction with current application rates and time,so that desired braze melting is achieved between the joined superalloycomponents 40, 50. Known flux agents and/or activators (e.g.,borane-dimethylamine) may be applied, as a separate material or mixedwithin the filler. The filler material may include superalloy materialbase metals of the type used to construct the superalloy components.

Referring to FIG. 6, the repair splice 50 is inserted into the repairedcomponent substrate 40, with the mating projecting portion 52 abuttingthe recess 44 along a contact surface 62, capturing any braze fillermaterial 60 therein. Thereafter the repair splice 50 and componentsubstrate 40 are compressed relative to each other, with pressure beingconcentrated along the contact surface 62. While the mating repairsplice 50 and component substrate 44 are being compressed, a known typeof resistance brazing heater 20′ passes current between electrodes 22′,24′, with heat transfer being concentrated along the contact surface 62.While the resistance heater of FIG. 6 places electrodes 22′, 24′ onopposing sides of the repair splice 50 and component substrate 44, thoseskilled in the art will appreciate that other electrode orientations maybe used, that cause resistance heat transfer between the projectingportion 52 and recess 44 along the contact surface.

Similarly, a continuous seam weld longer than the electrode 22′, 24′surface area can be formed by using longer, linear electrodes, rollingwheel electrode(s), a plurality of proximal electrodes oriented in anarray or moving the electrodes serially relative to the superalloysubstrates 40, 50. Similarly, multiple recesses 44 and projectingportions 52 may be formed in superalloy components 40, 50 andsubsequently joined by resistance heating tools sequentially and/orsimultaneously.

In performing the joining processes of the present invention, sufficientpressure and electric current are selectively applied to cause localizedmelting and affixation of the projecting portion 52 and recess 44 alongthe contact surface 62, but not excessive heat or pressure that willsignificantly alter material properties of the component substrate 40 orthe splice 50 at any significant distance from the contact surface.Accordingly, localized heating is stopped when desired braze melting andaffixation is achieved. Localized melting properties along the contactsurface 62 are affected by the recess 44/projection 52 profile depth dand width W, as those dimensions (along with length) impact appliedpressure per unit area and heat transfer flux. It is desirable toconstruct the contact surface profile so that its depth d is smallerthan its width W, and with angled side walls, so that excess meltedbraze material is extruded from the contact surface 62 juncture, asshown in FIG.7. A suitable recess 44 profile depth d to width W ratio is1:3.

After desired braze melting along the contact surface 62 is achieved,resistance heating ceases and the now mutually affixed componentsubstrate 40 and repair splice 50 are allowed to cool for subsequentremoval from the resistance heating apparatus 20′. The repair splice 50and extruded braze material along the contact surface 62 are conformedto the surrounding surface profile, restoring the repaired component 40to its original condition, shown in FIG. 8. The now repaired component40 no longer has surface damage, which is replaced by fresh repairsplice material 52, having substantially similar material properties tothe original substrate material. Specifically, surface hardness andstrength properties within the splice are substantially similar to thoseof the original surrounding material. Mechanical or thermally inducedstresses within the component 40 can be transferred across the contactsurface to the splice, due to their mechanically and thermally abuttingrelationship. Relative affixation between the repaired componentsubstrate 40 and the splice 50 along the contact surface 62 issufficient to maintain structural integrity.

The localized affixation along the contact surface 62 does notsignificantly negatively impact structural material properties of therepaired component substrate 40 and the splice 50. Limited post repairheat treatment (if any is required) minimizes—if not totallyeliminates—subsequent risk of repaired component strain age cracking.Thus, time and expense of superalloy component fabrication or repair maybe undertaken with the repair methods of the present invention, withoutundue risk of repair failure. In the power generation field, surfacecracked turbine blades may be repaired without the need to scrap andreplace them with new blades.

Different known resistance heating methods may be employed to join themating superalloy components. Suitable resistance heating methodsinclude resistance brazing, resistance braze diffusion bonding, andresistance transient liquid phase bonding. Resistance heatingjoining/bonding may be carried out in ambient air, or alternatively inisolated vacuum, inert gas or active gas environments. As noted above,the resistance heating joining/bonding may be performed with filleralloy, flux and/or activator compositions that are compatible with theselected heating process. In any of the selected resistance heatingmethods the overall objective is to achieve localized bonding betweenthe superalloy subcomponents along the contact surface 62 and ceasingadditional heat input, without significantly impacting the superalloymaterial properties within the generalized substrate of eithersubcomponent. Unlike known welding techniques that tend to liquefy therespective superalloy substrates (and thus negatively alter materialproperties making them susceptible to subsequent cracking), theresistance heating application in the present invention avoids grosschanges in the superalloy substrates—especially eliminating residualstresses associated with shrinkage typical during weld solidification.

When practicing the present invention, superalloy components can bejoined or repaired with known proven equipment. Damaged superalloycomponent material can be removed and repair splices fabricated byelectric discharge machining or other known metal cutting techniques.Known electric resistance heating brazing machinery and techniques maybe employed to affix repair splices to their mating recesses. Thesplice-repaired superalloy component external surface profile can berestored to original profile specifications by grinding or cutting therepair splice and surrounding contact surface to match the repairedcomponent's local profile.

While the structural joining of superalloy components exemplaryembodiments herein have been primarily described with reference tosuperalloy component repair, the same methods may be used to fabricatesuperalloy structures by joining subcomponents. For example, aprojection on one superalloy subcomponent may be affixed to a recess onanother superalloy subcomponent.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

What is claimed is:
 1. A method for joining superalloy structures,comprising: forming a recess in a superalloy component substrate havinga recess profile defined by the remaining substrate; forming a matingsuperalloy splice having a splice projection, with a projection profileconforming with the substrate recess profile along a correspondingcontact surface; interposing brazing alloy between the recess and repairsplice along the contact surface; inserting and capturing the splicewithin the recess, so that the projection and recess are in abuttingcontact along the contact surface; compressing the substrate and spliceprojection together along the contact surface at a selected pressure;conductively contacting the substrate and splice with separate electricresistance brazing electrodes; and passing current at a selected flowrate and application time period through the substrate and spliceprojection between the electrodes until brazing alloy melting occursalong the contact surface, and ceasing further current flow after thesubstrate and splice projection are mutually affixed to each other. 2.The method of claim 1, wherein the recess forming step is performed byelectric discharge machining.
 3. The method of claim 1, wherein duringthe recess forming step, the recess so formed comprises a blind recessformed partially within the component substrate thickness for engagementwith a mating projecting portion formed in the splice.
 4. The method ofclaim 1, wherein during the recess and splice forming steps the recessand splice projection mating profiles so formed only allowunidirectional insertion of the repair splice.
 5. The method of claim 1,wherein during the recess and splice forming steps the recess and repairsplice projection mating profiles so formed are planar.
 6. The method ofclaim 1, wherein during the interposing step the brazing alloy isselected from the group consisting of a powdered brazing alloy, solidpreshaped/preformed ring, foil or ribbon brazing alloy, granular brazingalloy, presintered braze material, or paste brazing alloy.
 7. The methodof claim 6, wherein the substrate and repair splice are constructed of asame material with substantially similar mechanical structuralproperties, affixed to each other with brazing alloy that upon itsmelting does not substantially change said structural properties.
 8. Themethod of claim 7, wherein the repaired component substrate is selectedfrom the group consisting of turbine blades and turbine vanes.
 9. Themethod of claim 1, further comprising conforming profile of an exteriorfacing surface of the repair splice with that of the surroundingsubstrate.
 10. A method for repairing a superalloy component,comprising: removing a damaged portion of superalloy component substrateand forming an excavated recess therein having a recess profile definedby the remaining substrate; forming a mating superalloy repair splicehaving a splice projection; with a projection profile conforming withthe substrate recess profile along a corresponding contact surface;interposing brazing alloy between the recess and repair splice along thecontact surface; inserting and capturing the repair splice within therecess, so that the projection and recess are in abutting contact alongthe contact surface; compressing the substrate and splice projectiontogether along the contact surface at a selected pressure with a pair ofopposed electric resistance brazing electrodes; and passing current at aselected flow rate and application time period through the substrate andsplice projection between the electrodes until brazing alloy meltingoccurs along the contact surface and they are mutually affixed to eachother.
 11. The method of claim 10, wherein the substrate and repairsplice are constructed of a same material with substantially similarmechanical structural properties, affixed to each other with brazingalloy that upon its application does not substantially change saidstructural properties.
 12. The method of claim 11, wherein during theinterposing step the brazing alloy is selected from the group consistingof a powdered brazing alloy, solid preshaped/preformed ring, foil orribbon brazing alloy, granular brazing alloy, presintered brazematerial, or paste brazing alloy.
 13. The method of claim 10, whereinduring the interposing step the brazing alloy is selected from the groupconsisting of a powdered brazing alloy, solid preshaped/preformed ring,foil or ribbon brazing alloy, presintered braze material, granularbrazing alloy, or paste brazing alloy.
 14. The method of claim 10,wherein the recess forming step is performed by electric dischargemachining.
 15. The method of claim 10, wherein the repaired componentsubstrate is selected from the group consisting of turbine blades andturbine vanes.